Formation flooding by sulphur dioxide for recovering oil and gas



IPTEl? XR 3935393 New. 21, W67 F. R. E. MULDER' 3,353,597

FORMATION FLOODING BY SULPHUR DIOXIDE FOR HECOVERING OIL AND GAS Original Filed March 10, 1964 5 Sheets-Sheet 1 on. k LOW PRESSURE 302 33 SEPARATOR s0 INJECTION PUMP i J i in. 1. j as FLOOD DRIVE PUMP I i I l l I l 'IN-VEWR FRITHO RUDGER ERICK MULDER F. R. E. MULDER FORMATION FLOODING BY SULPHUR 010mm: FOR

New. 2E, 39%? RECOVERING OIL AND GAS Original Filed March 10, 1964 5 Sheets-Sheet 3 BER'z'i SFANOSTOHE CORE- SWANHILLS CRUDE.

I40 700 I400 80 concentration PPM by weight in water d wa Amer gas 2 3 4 flushing time, hours 16 30 :E 53; umuauoi @8265 805 E :o 5;. x.

flushing time, minues INVENTOR FRITHO RUDGER ERICK MULDER F. R. E. MULDER FORMATION FLOODING BY SULPHUR DIOXIDE FOR REIJOVERING OIL AND GAS Original Filed March 10. 1964 5 Sheets-Sheet 4.

produced water, mhgas, mIxIO" flushing time, minutes produced wafer, ml; gas, mlxlO" produced wo1er,ml;gos,mlx|0" INVENTOR FRITHO RUDGER ERICK MULDER EM. 2 W87 F. R. E. MULDER FORMATION FLOODING BY SULPHUR DIOXIDE FOR RBCOVBRING 011. AND GAS Ongmal iiled March 10. 1964 5 Sheets 5heet 5 INVENTOR FRITHO RUDGER ERICK MULDER 597 da n-d mn @Ew" 37033 n r r 4 .1 e

a l. &w Ag if ?a.3:eni i HOV. 21 1 3,353,597 tapping the desired oil-bearing formation. When an im- FORMATION FLOODING BY SULPHUR DIOXIDE FOR REQOVERENG 05L AND GAS Fntho R. Ericlr Mulder, Calgary, Alberta, (Ianada, ns-

sagnor to Home Oil Company Limited, Alberta, Canada, a corporation of (Ianada Iontinuatlon of application Ser. No. 350,838, Mar. 19, 1964. This application Ila l. 36, 1967, Ser. No. 599,682

Claims priority, application Canada, Sept. 4, 1963,

r2 Claims. icl. 166-9) ABSTRACT BF THE DISCLOSURE backing drive which may comprise water alone or water acidulated by' a minor proportion of dissolved sulfur dioxide, to drive petroleum to a recovery Well, to generate in-situ carbon dioxide, calcium and magnesium bisulfites as well as surface-2ension modifying substances, all to improve displacement of petroleum from rock pores.

This application is a continuation of Ser. No. 350,838

filed Mar. 10, 1964, and now abandoned. This invention relates to improvements in methods of recovering oil from petroleum-bearing earth formations by injecting fluids thereinto under pressure to flush oil from the rock pores for recovery at one or more output wells.

Under the provision of 35 U.S.C. 1E9, this application is entitled to the benefit of the Sept. 4, i963, filing date of Canadian application Ser. No. 883,313.

More particularly, the invention relates to improved flooding processes for secondary recovery of petroleum employing sulphur dioxide in liquefied form as a flooding agent or as a constituent thereof. According to the invention sulphur dioxide may be injected either as a bank of primary drive medium or in solution in a suitable fluid to provide a drive agent for formation oil and connate water.

By the practice of the novel method of the invention large increases in recovery of petroleum from an oilbearing formation may be obtained, and in addition the permeability of dolomitic or limestone formations may be increased concurrently with the production of oil therefrom. In all known formations the injection of sulphur dioxide is found to aid in the displacement of oil from the rock, by physical processes not clearly understood. The chemical activity of sulphur dioxide, producing insitu compounds in the nature of surface-active wetting agents by reactions with petroleum constituents, may be partly responsible for the improved permeability of a reservoir rock to petroleum. Moreover the reduction in viscosity of crude oil having even a small content of sulphur dioxide dissolved therein aids in the migration of petroleum under drive conditions in a recovery operation.

It has become conventional practice to resort to the use of a drive fluid under pressure to mechanically sweep out the oil remaining in formation pores after natural gas pressure falls below a value at which an economical production rate is maintained. Secondary recovery is also regularly practiced early in the life of a producing field to maintain formation pressure. Normally according to the process, one or more input wells are drilled into the formation. or previously producing wells are utilized. having predetermined spacings from an output Well or wells miscible driving fluid such as water is employed, capillary forces and surface tension phenomena within the reservolr rock pores prevent recovery of more than a fracaion of the petroleum products emplaced. A bank of a liquid primary drive agent miscible with the petroleum and formation connale water is preferably introduced into the formation ahead of a water or gas backing driv'; agent. The bank of primary drive agent is restricted in radial extent due to the cost of the liquids employed. Liquefied hydrocarbon gases, chiefly propane, have hithereto been favored, followed by gas, fresh water or brine, the formation pressure being kept high throughout the drive to avoid gasification of the primary agent.

Normally the very large volumes and consequent high cost of a liquefied hydrocarbon primary drive agent, the low or even negligible recovery of such agent at the output wells, and the small increase in petroleum recovery obtained by such process, make such operations not aimgather successful.

I have found that a superior primary drive fluid useful for secondary recovery of oil from any earth formation including sandstone and calcareous, i.e. limestone and dolomitic, oil-bearing rocks, is either liquid sulphur dioxide alone, or a solution of sulphur dioxide in any fluid miscible or immiscible with formation fluids and petroleum constituents, such as a light hydrocarbon, certain organic liquids or water. Various alcohols, particularly ethyl alcohol and higher alcohols may be employed as vehicles. Moreover, sulphur dioxide may be carried in a composite solvent comprising various mixtures of such liquids.

Sulphur dioxide possesses physical characteristics which render it highly suitable and surprisingly efiective as a miscible drive agent, being liquid at relatively low pressures even at any temperature likely to be encountered in producing formations. The solubility of liquid sulphur dioxide in water and in petroleum crudes is high. Liquid sulphur dioxide in fact is soluble in water above a critical pressure which is well below bottom hole pressures usually encountered. It is soluble in light hydrocarbons such as propane under moderate pressure and with ethyl and higher alcohols.

Sulphur dioxide produces a so-cailed weak acid in aqueous solution. The chemical reactions between the sulfurous acid formed and a limestone or dolomite rock produce bisulphites and liberate carbon dioxide, the equilibrium of the reactions at usual reservoir conditions being highly stable. Frecipitation is therefore a rela'ively insignificant possibility, while any CO generated assists in reducing the viscosity of the crude in which it dissolves. The solution attack of the rock minerals may proceed according to the following reactions:

chemical activity of the ionized portions of sulphur di== oxide aqueous solution with constituents of the petroleum. The foaming of crude oil expelled from test cores under S drive with water backing suggests that surface tension changes result from such chemical actiyity which aid dioxide or primary drive solution containing sulphur dioxide is forced through the formation. Propane or butane may be used as a light hydrocarbon liquid solvent vehicle, preferably carrying a major weight proportion of sulphur dioxide. Methane or air may be used as gas backing drive agents or as intermediate drive agents followed by a water or brine drive. The bank of primary drive agent will inevitably become depleted of sulphur dioxide through reaction in aqueous solution phase with carbonate minerals, if present. while dilution with connate water in the pores of the formation will increase as the bank advances. In carbonate rocic reservoirs, the gener' tion of in-situ carbon dioxide is advantage sustaining formation pressure and lowering petroleum viscosity, and little if any sulphur dioxide will appear at a recovery well. After water breakthrough at the input well or wells, the drive is continued to recover oil and water together, until an economic limit for the water/oil ratio is reached. Such ratio will be significantly larger than 1:1, and may approach :1 or higher ratio depending on the cost of the flooding operation and the rates of flow from output wells.

In a modification of the invention, where the formation permeability is sufiiciently high upon completion of the drive of the primary bank, the bottom hole pressure is reduced to a few pounds per square inch or even down to a few ounces, to allow gaseous sulphur dioxide, petroleum gases, methane. air, carbon dioxide, etc. to propel the fluids including oil out of the formation, in a blow/down operation. Such operation may be effected at the input well or wells also, in addition to recovery at output wells.

The flooding drive may be repeated as often as de sired, and in carbonate rock reservoirs a succession of drives may be necessary where the formation is initially tight, Le. has a low permeability to oil, so that the permeability at the output wells may be raised to a practicable value for higher production rates, as disclosed in my co-pending application Ser. No. 350,837 filed Mar. 10, 1964, now US. Patent No. 3,326,289.

in carrying my invention into practice for effective oil and gas recovery from a petroleum-bearing formation, and particularly from partially depleted petroleum reservoirs, the requisite amount of sulphur dioxide may be introduced by suitable pressure pumps into the well bottom of an injection well or wells penetrating the formation'. The sulphur dioxide may be pumped as dry gas which is led by a small diameter line of tubing down a well casing, the casing being packed above the discharge point to prevent any sulphur dioxide from entering the upper part of the casing. At typical formation pressures, the sulphur dioxide liquefies under moderate input pressures and such liquefaction process is continued until a sullicient bank of the liquid has been injected.

Where sulphur dioxide is initially procured as a stored liquidin pressure storage tanks the liquid may be pumped either alone or with a solvent vehicle into the formation. If it is intended to use a solvent in which sulphur dioxide is soluble, for example water, propane, butane or alcohol, this solvent may be injected either before or after the injection of liquid sulphur dioxide or alternately therewith, allowing the substances to dissolve into each other at their interfaces.

Since pure dry liquid or gaseous sulphur dioxide is non-corrosive, it is preferable to inject the material by a small diameter tubing directly into the well bottom,

and to comingle and dissolve any solvent vehicle in the well bottom by pumping such vehicle down by another tubing. The latter, may advantageously be the well casing itself, in order to avoid corrosion of the casing above the tubing-casing packer employed to close 0d the upper part of the well near the bottom. The backing drive medla are then pumped down the casing at a desired rate, to force the sulphur dioxide-rich primary band radially outwardly from the input well or wells through the formatron.

The volume of sulphur dioxide liquid injected will generally be a small fraction of the total pore volume of the formation to be flushed, so that "he advancing bank will measure from a few inches to a few feet in radial extent at a distance of 50 to 100 feet from the in jection well. The volume required may be computed, using a factor in the range 0.001 to about 0.3 times the pore volume of the iormation to be swept. The injection pressures may be anywhere in the range from 100 to about 5000 p.s.i.g. in the formation, depending on the character of the rock, formation depth and formation pres- E:-r.

A water or brine backing drive may be acidulated by dissolving a small percentage of sulphur dioxide therein, ranging from about one part per million by weight to several hundred thousand parts per million, to reduce the work of pumping and in carbonate rock formations to leach flow passages and channels slowly along the formation. To produce the largest possible fraction of the oil contained in the formation, the water or gas drive is completed while maintaining a bottom hole and formation pressure at least of the order of 560 pounds (gauge) until the water/oil fluid ratio of liuids produr ed exceeds about 1:1 or if warranted, 10:1 to 20:1. Jn conclusion of the final flooding if the formation pressure may be lowered, the bottom hole pressure at the recovery wells is reduced as by pumping at the level of the formation and the formation pressures are allowed to fall gradually until sulphur dioxide gas begins to be evolved with the fluids recovered. Additional water backing drive may be injected until a further limiting ratio of water/oil production is reached. 1

The invention may be the better understood by reference to the test data and tables hereinafter presented and to the accompanying figures of the drawing forming part of this specification, wherein: FIGURE 1 is a diagram showing one form of apparatus for injection of sulphur dioxide into a formation through an injection well and for recovery of oil and gas from an output well;

FIGURE 2 is a graph showing solubility of sulphur dioxide in water and a family of bubble-point curves for the mixture, and also showing solubilities of tw. types of crude oil for 50 FIGURES 3 through 7 are graph representations of test results obtained on various limestone cores in laboratory injection runs with a primary bank of liquid SO; and water backing drive;

FIGURE 8 shows test data in graph form of improvement in recovery of diesel fuel and of a crude oil from an artificial non-reactive alundum core;

FIGURE 9 shows test data for improvement in recovery of oil from a Berea sandstone core using an aqueous solution of S0 and FIGURE 10 is a graph showing improvement in recovery of a heavy crude from a sandstone core under drive by an aqueous solution and also showing data on surface tension of ellluents and interfacial tension between crude oil and water recovered from the core.

An installation for the injection of sulphur dioxide and recovery of oil and gas from a formation, employing one input and one output well is illustrated by FIGURE 1. wherein an input well 10 penetrates an oil-bearing formation R as does an output well 11 spaced suitably therefrom by a distance which may be from about 10G feet to a half mile or more. The wells have casings "l2 entering or communicating with the formation and have a cemented seal 13 placed between the casing and the earth formation J overlying the oil-bearing formation to seal off the latter from layers above it.

injection well includes a tubing 14 terminating in the well bottom and connected with a sulphur dioxide supply 15 through pump 16. The annular space 17 is connected through pump 18 with a water supply or a supply of gas or other fluid (not shown) fed into line 19.

The chemical activity of weak aqueous solutions of sulphur dioxide requires that all containers, pumps and duct: with which it will come into contact should be made of materials capable of withstanding its corrosive effects. Pure dry sulphur dioxide gas or liquid is not corrosive, and may be safely handled in copper lines with copper or brass fittings, but very small amounts of water vapour in $0 gas, or very small amounts of dissolved sulphur dioxide in water, are highly corrosive mixtures. Certain stainless steels are incapable of resisting corrosive attack by these mixtures. Valve seats and plugs, piping, fittings, gauges and control. elements may be fabricated of or lined with lead. plastic, or with an alloy such as Hastelloy, Stellite or Monet metal, which are resistant to corrosive action by 50 Oil and gas recovered at the output well 11 is led into a separator 20, which preferably reduces the pressure on the fluids to a low value to remove gaseous materials. Recovered water may be fed by line 21 to supply the pump 18 for backing drive, while any released gas, including gaseous sulphur dioxide, is led by line 22 and compressor 24 to a high-pressure separator 23. Liquefied sulphur dioxide is returned to the supply 15 for re-use.

Where conditions require, a pump 25 such as an electrically driven unit powered from a line 26 may be operated to lower the formation pressure as desired. to a pressure below the vapour pressure of S0 at the formation temperature. The pumped products may be passed through a run of corrosion-resistant tubing 27 inside the casing 512, particularly a recovery operation from a sandstone reservoir, where the acidity of the waterwould rapidly destroy the casing.

In an operation where water recovered from an output well carries substantially no dissolved 80 as when recovering products from a carbonate rock reservoir, the injection may be made into the annular space 17 inside casing 12 of a well 10 and any additional agents desired may be introduced by line 19. Where the water is sour the water backing drive preferably is injected through tubing string 14, and space 17 is kept filled with an inert substance such as methane, air, propane, or sweet water to prevent entry of the corrosive solution from below.

It will be apparent in the solubility diagram, FIGURE 2 that at moderately high temperatures such as exist in deep reservoirs the recovered fluids have a relatively low solubility for S0; at atmospheric or sub-atmospheric pressures. Consequently a large part of any 50 recovered from a sandstone reservoir will be stripped in low-pressure separator 20. Referring to the 56 C. line in the diagram, at sub-atmospheric pressure the efiluent water from well "11 is capable of holding in solution only a fraction of a gram of 80;, per 100 ml. volume whereas above a pressure of about 560 pounds per square inch in the reservoir any amount of liquid SO remains in solution equilibrium. Even in a very hot reservoir rock, i.e. near 100 C., a pressure in excess of about 560 pounds per square inch prevents gasification of SO which remains in a single phase solution with the water.

The data in FIGURE 2 also relates to the solubility observed for mixtures of liquid $0 in two types of crude oils over a range of pressures. The Turner Valley crude is a light, sour crude containing hydrogen sulfide and. also a considerable amount of relatively insoluble waxes, having a viscosity at 150 F. of 1.5 centipoises.

The Swan Hills crude is a dark sweet crude containing 6 some wax sediment with a viscosity of 2.5 centipoises at r.

On mixing of each of these crude oils with liquid SO; and sampling the mixture under isobaric and isothermal conditions and analysis for S0 content, the plots of solubilities were obtained as graphically shown, wherein the concentration of the solution is expressed in pounds of SO; per barrel of 35 imperial gallons of crude oil. The Turner Valley crude has an upper limit of solubility for liquid 50 over a range of very high pressures as may be encountered in the reservoir, of about 110 pounds by weight per barrel of 35 imperial gallons of the crude. Tests with Swan Hills crude oil show a slightly higher solution capacity of crude oil for 50 about 143 pounds per barrel at 71 F. Other crudes including very heavy, tar-like crudes have been found to have a considerable although lower capacity for solution of S0 The system (propanezSO; liquid) is a single phase solution at the elevated pressures and typical reservoir temperatures of deep wells. The solubility curve is not shown, since at any pressure above the vapour pressure of 50-. liquid for a given temperature the curve is unde- Site solubility relation of S0; with liquefied ii ;-ht by 'ocar-bons is similar, exhibiting complete miscibil ty.

in FIGURE 3, curves are plotted from oil production, gas production, and water injection data obtained from tests carried out on a core to demonstrate the practice of the invention. These data are based on the test carried out as described in Example I which follows. Similarly obtained data are plotted in FiGURES 4 to 7 inclusive, representing results of Examples 11 to V inclusive.

Example I A section of limestone formation cored from the Car stairs Alberta field, well No. 10, at about the 8,250 depth. was cut to a diameter of 3.5 inches and a length of 8 inches shortly after coring. This core was mounted in a 5 /2 inch diameter steel well casing length and the cylindrical surface of the core/was sealed by an inert filler occupying the annular spiicebetwecn the core and the casing to prevent migration of the core liquids past the walls. The ends of the core were cut, ground, and sanded smooth llush with the ends of the casing.

The casing containing the core was then mounted between two high-pressure corrosion-resistant steel flanges with fiat neoprene gaskets at each end to prevent leakage. The flanges were bolted with steel tie bolts thus compressing the gaskets and steel casing to form a tight seal. The flanges were provided with central apertures connecting with stainless steel tubing. A Heise pressure gauge was mounted and manifolded to the tubing at each end of the core so that both upstream and downstream core pressures could be measured. Further input lines and valves were connected to supply selected fluids from various sources and to receive effluents.

The core was flushed with about eight times the estimated pore volume, using the commercial solvent Varsol followed by propane under p.s.i.g. inlet and 75 p.s.i.g. outlet pressures, for 6 hours. The core was then dried for three days by flowing dried air through it at roc'm temperature.

The cleaned core was evacuated by a vane type of mechanical vacuum pump and distilled water allowed to enter the inlet and under the gradient of one atmosphere, with the connection to the vacuum line sealed off. The amount of water taken up was measured and recorded as the effective pore volume. At the same time the water permeability of the core was determined by measurement of the how rate through it under specific gradients, and was found to be 2.15 millidarcies, hereinafter abbreviated md.

The water-saturated core was flooded with about ten pore volumes of a clear refined light lubricating oil with a viscosity of 13.6 centipoises at 60 F., the effluent being carefully collected. The initial oil-in-place volume was oil, water, and gas was recorded.

3 determined as the amount of efiluent water displaced by the oil flushing. u

Pure water under input pressure of 1300 p.'s.i.g. was then injected into the core at a constant pressure drop of 350 p.s.i.g. across the ends, providing a gradient of 43.75 pounds per inch length. The volumes of oil and water recovered iii a separator were recorded at intervals for six and a half hours, during which time the temperature was held at 75 F. until a production water/oil ratio of 20:1 had been reached.

The core was reilooded with the same type of oil and the amount of efiluent water was recorded, the oil now held by the rock being computed from the original oilin-place quantity less the recovery from Run 1, plus the volume of water expelled on reflooding.

A second test run consisted in the steps of first injecting liquid into the oil-saturated core under a pressure of 1300 psig, then flooding the core with water when the S0 injection amounted to 0.03 of the pore space. The temperature was held at 73.5 P. The production of the gas volume be measured by a gas hurette at ten...- ressure. it 5141'- production of nearly 9% of the initial chin-place occurred, within a halt minute. The flooding water was observed to break through the downstream end of the core in about 17 minutes. while the oil recovery tapered 03 during this interval. The water throughput reached a water/oil ratio of :1 after two hours and forty minutes at which time the amount of recovered oil was a little less than for the proceeding test under pure water hood of Run 1.

Thewater drive was shut off. and the efiiuent end of the core was depressurized to atmospheric pressure by in crements, while the gas, oil and water produced during the blowdown was recorded. More than half of the total oil recovered was produced on blowdown. and very little water or gas- All production of oil on biowclown occurred in the last stages of depressurization, especially inthe range of outlet pressures from 50 p.s.i.g. down to 0 p.s.l.g.

vThe amount of oil produced from the core by the S0; liquid flood with water backing was 67% of the oil initially in place as against 42.5% recovered by straight water flood, i.e. an improvement of 50% in production. Some CO gas was liberated but no sulphur dioxide was apparent in the gas. At breakthrough a slight yellowing of the etiluent water was noted.

Still larger quantities of liquid 50;; injected as a bank of primary flooding agent are beneficial, as demonstrated 'by a further experimental secondary production test in the laboratory wherein a liquid volume of 50; equal to I 0.1 pore volumes of the core was injected, as set out in Another limestone core, taken from the Home Regent well A of the Swan Hills Alberta field at'a depth of 8272 feet, was cleaned using n-pentane, and dried as in Example" i, and its porosity and permeability to water were determined as for the core of Example I. These measured 12.8% and 30 md., respectively, the latter being a relatively high permeability for limestones.

The core was saturated with water and flooded with Marcel GX refined oil as in Example 1. Run 1 was made using distilled water drive injected under a pressure of 950 p.s.i.g. against a back pressure of 920 p.s.i.g., providing a gradient of 7.4 pounds per square inch per inch of core length. The water flood produced oil rapidly for the first four minutes, after which the water broke through. The oil production rate fell to about a third of its value prior to breakthrough, and held steady, until the run was discontinued at a water/ oil production ratio of 20:1 which was reached after a total flooding time of 19 minutes. The recovery amounted to 53.5% of the initial volume 8 of oil in place. The etliuent water appeared milky, showing evidence of some cmulsification.

The core was cleaned and dried in prcpaartion for Run 2 and the water permeability was observed to have dropped to 25 rnd. as a result of the water flood of Run 1, as was expected.

An injection of liquid SO; amounting to 10% of the pore volume was injected into the core under inlet pressure of 975 p.s.i.g. at 73 5., while outlet pressure was held at 950 p.s.i.g. The water flow was very slight for four minutes, after which it increased to a high rate, exceeding the fiow of oil.

Depressurization of the effluent side of the core with the inlet side closed off was regulated by increments. The blowdown produced as much oil in one minute as had been recovered in the first six minutes. The blowdown recovery oil was dirty and brown in colour, apparently suspending fine sediments from the rock.

After immediate cleansing of the core, it was found that the water permeability had risen to 250 md., indicating he contact with S0; liquid in the presence of water opened up flow channels. it will be seen from an inspection of data in FIGURE 4 that very rapid displacement of the flooding tluids and rapid recovery of oil resuited from the process.

The injection of S0 liquid under temperature and pressure conditions of a deep formation into a core thereof saturated with an actual crude oil produced from the same formation, was shown to be highly successful in increasing secondary recovery of the oil as will next be described in Example Ill.

Example III A core 5 /2 inches in length taken from the Home Regent well 3" of the Swan Hills field in Central Alberta at a depth of 8230 feet. was cleaned and dried as in Example ii, and saturated with pure water. The permeability was measured as 4.0 md., and the porosity as 13.3% of the core volume.

The core was saturated by flooding with a Swan Hiils crude oil. In Run 1 as shown in FIGURE 5, the rer'reme water fiood showed a high rate of oil and gas recovery when applied at inlet pressure of 1100 p.s.i.g. and a pressure drop between the ends of the core of pounds per square inch. Water breakthrough did not occur until 60 minutes after the test began, and rose rapidly, appearing as an emulsion with the oil. Deprcssurizing the effluent side of the core with the water drive shut off produced a little gas but no significant amount of fluid.

The core was reflooded with the crude oil, and Run 2 was made, consisting of the step of injecting a volume of liquid S0 equal to 0.l0 pore volume, followed by the step of forcing water through the core sample. An imn-e diate production of oil surged through, amounting to 17% of the initial oil-in-place. and the rate of oil flow steadied for six minutes before rapidly rising. Water broke through after eight minutes. which contrasts with the breakthrough at 57 minutes for the straight water flood. After 10 minutes, the core was depressurized at the effluent end by increments with the inlet end sealed off which rapidly produced oil in an amount exceeding the recovery in the entire water flood of Run 1. The total oil recovery from the rock was astonishingly high, amounting to 77% of the oil initially in place and its recovery was eflected in a short space of time, indicating that a high flow rate would be reached under field conditions. The water produced was yellow, and the gas rate following breakthrough was high, some sediment appearing in the oil.

After Run 2 the effective water permeability of the rock sample was found to be 4.2 md., indicating that no substantial increase in permeability or channeling had occurred. It is significant that the how rates, as indicated by a total flooding time of 17 minutes, were greatly increased as compared with Run 1, suggesting that frecrcr flow paths had been opened up.

The beneficial effects of injection of SO; as a bank of primary drive agent into limestone cores for water flood drives ofa dark sour crude oil containing hydrogen sui- Example I V A limestone core from the Home Harmattan well of the Harmattan field, taken from the 8570 foot depth, was cut to a length of 7 inches. The core was cleaned with Varsol solvent and dried, then water saturated as described above. A porosity of 14.5% and a water permeability of 6 md. were measured. The core sample was heated to 150" F. and was oil-flooded with Turner Valley crude oil at that temperature.

The plot of Run 1 shown in FIGURE 6 shows pure water flooding production results for an inlet injection pressure of 970 p.s.i.g. and a drop over the core length of p.s.i.g. The curves show water breakthrough at 37 minutes, a steady slope for the oil and gas production quantities, a rising water throughput rate, and a large subordinate oil and gas recovery. Some blowdown recovery was obtained, with a total recovery of about After refiooding with Turner Valley crude oil, Run 2 was carried out, consisting of an initial injection of 0.05 pore volumes of liquid S0; at 970 p.s.i.g. follower by pure water backing drive. The initial rates of oil and gas production were high, but decreased and paralleled the production curves of Ron 1. The rate of oil production increased sharply just before breakthrough anddecreased immediately-after water breakthrough, although the water produced closely paralleled that of Run 1. Blowdown recovery was 20% of the rock content.

On cleaning and resaturation with water, it was found that the water permeability of the rock has risen to 9.35 md. from 6.2 md.

This test established that even without the production obtained on blowdown, the injection of a liquid 50; bank produced 15% more of the oil initially in place than did the pure water flood, and that the total production recovered from the reel; including blowdown was twice that recovered by water flooding alone.

In some types of limestone reservoir rock an injection of 5 pore volume percent of liquid 50 under temperature and pressure conditions of a deep formation followed by a water flood may be effective to produce as much as 70% of the oil initially in place including blowdown recovery, as compared with a maximum recovery of about 40% using conventional water flood, as shown by the following Example V.

Example V onder the same high pressure (950 p.s.i.g.) injection as during oil saturation, with a pressure drop across the lcngth'of 8 inches of 10 pounds per square inch. Data appear in FIGURE 7.

At water-breakthrough the oil production showed an abrupt decrease in rate but continued to produce a substantial subordinate production. A relatively high water production rate was noted and the test was carried to a water/oil production ratio of 5:1'.

The core was refloodcd with the crude oil and Run 2 was made, starting with an injection of liquid under the simulated deep well conditions. The primary injection immediately produced 8.5% of the oil initially in place, as an amount of S0 equal to 5 pore volume percent was forced into the core. The water backing drive was carried on until a water/oil ratio of production of 5:1 was reached. Considerable gas was produced as a foam with the oil, and water breakthrough was much delayed. On blowdown a substantial amount of oil was produced, yielding it high total recovery. The permeability of the core to water on re-cleaning and saturating had decreased to 4.7 md.

FIGURE 8 shows data obtained by tests according to the method of the invention carried out on non-reactive rock, using an aqueous solution of sulphur dioxide at various concentrations, and alternatively using liquid S0 as a primary drive agent followed by water backing drive.

The experiments were carried out on a synthesized p0- rous rock made of sintered aluminum oxide (Alundum) particles, with a pore volume equal to 24.9% of its bulk and having a high water permeability of 325 md. The core measured 18 inches long and its cross-sectional area was 0.82 square inch. Injection of all fluids was made under test conditions of a low temperature reservoir at moderate pressures in excess of 1000 p.s.i.g. and 78 F. 1' er initial water saturation, a first series of tests was e diesel fuel as the emplaced oil, having a viscosrty of. 5.95 centipoises. The flooding was regulated to force iZO cc. of fluid per hour into the core. Each test of a series was made using a different concentration of S0 dissolved in flood water, covering the range 0 to 20 percent by volume S0 mixed with water.

An initial test with pure water flood yielded 48.9% of the emplaced oil at breakthrough and 55.1% at a 20:1 water/ oil production ratio. On addition of $0 the recovery at breakthrough and at the limiting water/oil ratio improved, the rate of improvement being less above 2% of added 50;, but some improvement being still obtained by increasing the strength even at a; concentration of 20%, which was a single phase solution. When such high concentration was used a breakthrough recovery of 64% of the cmplaceo' oil was obtained, and at the 20:1 water/oil production ratio the total recovery was 71.7%.

A second series of tests using Swan Hills crude oil and heated cores, with a range of concentrations of aqueous solutions of 50 as flooding fluids also at 150 F., ranging from 0 to 10 percent S0 by volume, showed consistent improvement in recovery as the concentration increased. Ultimate recoveries of 68.7% of the oil emplaced were measured at the breakthrough' point and 78.5% at the 20:1 ratio of watcr-to-oil produced, with an applied water flood containing 10 percent S0 by volume.

The same core was used in further tests using diesel fuel again as the emplaced oil, one test using an initial injection of 0.2 pore volume of liquid S0 followed by water drive, and a second test using 1.0 pore volume of liquid S0 followed by water drive.

It will be apparent from the data obtained as shown in the diagram for the 50; concentration. that recovery at breakthrough is lower than when using pure water or an aqueous solution. and that a slightly higher ultimate recovery is obtained for this oil with the 1.0'pore volume bank, and water backing drive. For a given reservoir sand it may obviously be established by experiment Whether it is more economical to use a bank of liquid S0 as primary drive agent followed by aqueous or gaseous drive, or to use an aqueous flood containing dissolved 50;.

A low viscosity crude may, for example, be recovered by a flood water drive having relatively low content of dissolved $0: from a Berca sandstone or equivalent rock, as shown by FIGURE 9. The graph relates data obtained with a cylindrical core having length and diameter dimensions respectively of 11 inches and 3 inches. The core was mounted for axial flooding with its cylindrical face scaled, and was injected at a constant high input pressure and a constant rate of 240 cc. per hour through one end face with various concentrations of sulphur dioxide in water. The rock porosity Was 18.03% and the computed pore volume was 18.9 cubic inches. Its permeability to coveries of crude per cm. at 26 C.

water was 1.223 md. After an initial cleaning and water flood saturating the rock. a sweet crude oil, obtained from the Swan Hills, Alberta field. having a viscosity of 3.5 cp. at the test temperature of 80 F., was cmplaced in the sandstone by flowing about 6 pore volumes of the crude theretltrough. At the beginning of each test the oil content was about 71% of the pore volume.

The graph shows that for increasing concentrations of 50 the petroleum was increasingly displaced in preference to the connatc water. as demonstrated by the fact that only 30% of the initial oil-in-place was flushed out at breakthrough of connnte water irom the discharge face of the core under a pure water flood. Total oil displacement was higher with acidulated water flooding than with pure water at any water/oil ratio. It is significant that when 50 is added to flood water in a proportion as low as about one part per thousand by volume, an improved recovery amounting to as much as 50% of total recovery with a pure water flood may be gained, and a lower pressure gradient may reduce the of injection by a large factor.

It has also been found the use of an aqueous sol'ution of sulphur dioxide is exceptionally advantageous in secondary recovery of a depleted or depressurized oil reservoir in which the crude oil has a high viscosity. Certain crudes resemble plastic solids although being tech.- nically liquids, and have such high viscosity that they are substantially deadf' i.e., have little or no capacity for flowing. As might be expected, such crudes respond poorly to water drive or to drive with known flooding media. FIGURE 10 shows test data for laboratory trials using an unconsolidated sand core having a permeability of 2700 md. and a porosity of 36.4%, the curves for Run 1 showing water production and oil production under pure water flood for a Lloydminster, Alberta crude pumped from a gas-free field having a viscosity of 2720 centipoises at 30 I IF. The curves tor Run 2 show secondary recovery under an applied waterftood containing 10 percent by volume The abscissa of the graph is plotted as pore volumes of flooding medium injected. it will be apparent that refor up to about one pore volume of fluid injected are about the same for either flooding medium, but that increasing flooding with acidulatcd water particularly at or above 2.0 pore volumes input.

The graph also includes plots of the variations of surface tension of the recovered oil and produced water effluents, against input volume of flooding medium, and

also a plot of the variations of interiacial tension measured for the produced oil with water. it will be observed that the surface tension of the crude, normally 33.5 dynes test temperature. rose slowly with increased production to 36.0 dynes per cm. and fell to 3.0 on blowdown. The surface tension of flooding water containing 10 volume percent of S0 liquid in solution, at a pH of 0.88 was measured as 52.2 dynes per cm. at one atmosphere. The water at breakthrough had a lowered surface tension of 31.7 dynes per cm., which probably contained some *of the connate Water saturating the core, as the pH was 4.60. The surface tension rose abruptly after one pore volume had been injected and the effluent pl-I fell to 0.91.. The water on blowdown was found to have a relatively high surface tension of 64.7 dynes per cm. although it was quite sour at a pH of 1.07.

The interfacial tension of the crude with pure water was measured as 20.6 dynes per cm.. and with the acidulated flood water at 18.4 dynes per cm. The ellluents exhibited a steadily rising interfncial tension, commencing at 18.0 and reaching 33.2 dyncs per cm. The foregoing data, while not conclusive evidence of any specific phenomena, may be interpreted in part as showing a substantial reduction in the resistance offered by a crude t0 displacement through the sand in accordance with the drop in surface tension of the ZlClLllllLllCd water from 52.2

a much higher recovery is made with ill dynes per cm. at the input to 32.9 or lcss'in the effluent. Undoubtedly the physical phenomena attending the mimotion of connatc water, oil constituents, and flooding medium through and along the tortuous and restricted pore openings are complex and probably inexplicable.

The interpretation offered for the foregoing data is in no way intended to confine the scope of the present invention, which has been shown to provide substantial and even unprecedented improvemets in recovery of widely difiering types of petroleum from various kinds of reservoir rock.

It has been shown in the foregoing description that the injection of sulphur dioxide as a pure liquid. or as a solution in any proportion in a bank of water under pressure or a bank of liquefied light hydrocarbon vehicle as a primary drive agent and followed by a backing drive medium, or as a solution in a water flooding drive alone, is fiicacious for the purpose of increasing the ultimate recovery of oil from the reservoir rock. While the invention has been particularly described for a recovery ope'atic using spaced input and output wells, it is to be und .rd that the invention is not limited to such recovery method, but may also be practiced with a single well. Such single well may be either a newly drilled well penctrating an oil-bearing porous formation or a well from which production has been obtained for some time, and the formation pressure may be high or low including near-atmospheric pressure conditions. The method of the invention may be practiced as described, injecting either liquid S0 or a vehicle solvent containing SO in a single phase solution followed by a backing medium, or by injecting an aqueous solution of S0 alone. When the amount of injected backing medium or aqueous solution has built up the formation pressure, particularly where a gas hacking drive such as methane or air follows the primary drive agent or an aqueous flood containing a minor volume proportion of 50 the formation fluils including any oil and gas may be recovered by outflow from the well, using pumping means it" necessary to recover the media.

I claim:

1. In the secondary recovery of petroleum from an oil-bearing earth formation penetrated by at least one injection well and by at least one recovery well spaced therefrom, the two step method which comprises (at) introducing liquefied sulphur dioxide into the formation through an injection well to form a bank of liquid sulphur dioxide in the formation adjacent said well and (b) thereafter introducing through said injection well a backing fluid comprising water acidulated by a minor proportion of dissolved sulphur dioxide into said formation immediately behind said bank sufficient to drive through the formation to said recovery well, said bank and petroleum displaced from the formation and without cyclical repetition of steps (a) and (b), re-

covering said petroleum through said recovery well,

2. The method as defined in claim 1 wherein the sulphur dioxide is introduced at the injection well head in gaseous form and the formation pressure at the bottom of said injection well is maintained in the range from 200 to about 5,000 p.s.i.g.

3. in the secondary recovery of petroleum from an oil, bearing earth formation penetrated by at least one injection well and by at least one recovery well spaced there from, the two step method which comprises (a) introducing a primary drive fluid consisting of a solution of sulphur dioxide in water into the fo mation through an injection well to form a bank of recovering said petroleum through said recovery well.

4. The method as defined in claim 3 in which said bank has a volume equivalent to about 0.001 to about 0.3 pore volume of the formation swept by said bank.

5. The method as defined in claim 4 in which said bank is a solution of from 1.000 to about 200,000 parts per million of sulphur dioxide in water.

6. The method as defined in claim 4 in which the back ing fiuid is water. 1

7. The method as defined in claim 6 in which the water is acidulated by a minor proportion of dissolved sulphur dioxide.

8. In the secondary recovery of petroleum from an oil-bearing earth formation penetrated by at least one injection well and by at least one recovery well spaced therefrom, the two step method which comprises (a) introducing liquefied suiphur dioxide into the formation through an injection well to form afoank of liquid sulphur dioxide in the formation adjacent said well and (b) thereafter introducing through said injection wet;

.a backing fluid comprising water acidulated by a minor proportion of dissolved sulphur dioxide into said formation immediately behind said bank surficient to drive through the formation to said recovery well, said bank and petroleum displaced from the formation and recovering said petroleum through said recovery well.

9. The method as defined in claim 8 wherein the sulphur dioxide is introduced at the injection well head in gaseous form and the formation pressure at the bottom of said injection well is maintained in the range from 200 to about 5,000 p.s.i.g.

i4 10. In the secondary recovery of petroleum from an oil-bearing earth formation penetrated by at least one injection well and by at least one recovery well spaced therefrom, the two step method which comprises 5 (at) introducing a primary drive fiuid consisting of a.

solution of sulphur dioxide in water into the formation through an injection well to form a bank of said solution in the formation adjacent said well and (b) thereafter introducing through said injection well an aqueous backing fluid only into said formation immediately behind said bank sufficient. to drive through the formation to said recovery well, said bank and petroleum displaced from the formation and recovering said petroleum through said recovery well. 11. The method as defined in claim 10 in which the backing fiuid is water.

12. The method as defined in claim 11 ip which the water is acidulated by a minor proportion of dissolved sulphur dioxide.

References Cited UNITED STATES PATENTS 2,359,818 11/1952 Hall et a1. 166-9 2,910,123 10/1959 Elkinsetal "166-9 2,968,350 1/1961 Slobod et a1. .66--9 25 3,120,262 2/1964 Archer 166-9 3,249,157 5/1966 Brigham et al. 166-9 FOREIGN PATENTS 0 696,524 9/1953 Great Britain. 0

OTHER REFERENCES Muskat: Physical Principles of Oil Production, 1st ed., McGraw-Hill Book Co., New York, 1949, pg. 703 relied STEPHEN 1. NOVOSAD, Primary Examiner. 

1. IN THE SECONDARY RECOVERY OF PETROLEUM FROM AN OIL-BEARING EARTH FORMATION PENETRATED BY AT LEAST ONE INJECTION WELL AND BY AT LEAST ONE RECOVERY WELL SPACED THEREFROM, THE TWO STEP METHOD WHICH COMPRISES (A) INTRODUCING LIQUEFIED SULPHUR DIOXIDE INTO THE FORMATION THROUGH AN INJECTION WELL TO FORM A BANK OF LIQUID SULPHUR DIOXIDE IN THE FORMATION ADJACENT SAID WELL AND (B) THEREAFTER INTRODUCING THROUGH SAID INJECTION WELL A BACKING FLUID COMPRISING WATER ACIDULATED BY A MINOR PROPORTION OF DISSOLVED SULPHUR DIOXIDE INTO SAID FORMATION IMMEDIATELY BEHIND SAID BANK SUFFICIENT TO DRIVE THROUGH THE FORMATION TO SAID RECOVERY WELL, SAID BANK AND PETROLEUM DISPLACED FROM THE FORMATION AND WITHOUT CYCLICAL REPETITION OF STEPS (A) AND (B), RECOVERING SAID PETROLEUM THROUGH SAID RECOVERY WELL. 